1. Field of the Invention
This invention relates to a method for producing non-permeable carbonaceous formed bodies.
In addition to good non-permeability to gases and liquids, non-permeable carbonaceous formed bodies possesses excellent properties such as low electric resistance and strong resistance to chemicals, so that they are expected to find wide industrial applications in electronic, neuclear and aeronautic fields, and above all the above-mentioned properties are suitable for use as separating sheets in phosphate type fuel cells which are attracting attention of many concerns in these days.
2. Description of the Prior Art
The phosphate type fuel cell consists of, for example, a plural number of layered unit cells each having a matrix impregnated with phosphate serving as an electrolyte, a pair of porous catalyst-carrying electrode plates grippingly holding the opposite sides of the matrix, and separating sheets laid on the outer sides of the electrode plates. More particularly, in the case of a fuel cell of the so-called ribbed electrode type, each electrode plate is provided with ribs on the side of the separating sheet to supply a fuel gas or an oxidizing gas to the grooves between the respective ribs. Namely, a gaseous fuel such as hydrogen gas is fed to the grooves on one plate while a gaseous oxidizer such as air and oxygen is fed to the grooves on the other plate, to effect the cell reaction. Accordingly, the separating sheet which is supplied with a fuel on one side and with an oxidizing agent on the other side thereof is required to have excellent non-permeability to gases to prevent mixing of these gases. Besides, it is required to have high electric conductivity along with high compessive and bending strengths in the form of stratified thin sheets, in order to function as a collector of the fuel cell which is constituted by layered unit cells as mentioned above.
However, non-permeable carbon moldings which have thus far been known in the art are all found to be unsatisfactory in the above-mentioned properties. For example, Japanese Laid-Open Patent Specification No. 54-20991 discloses a method of preparing carbon moldings consisting substantially of glassy carbon alone by mixing and kneading fine powder of thermosetting phenol resin and a primary phenol-aldehyde condensate and, after molding to shape, burning the moldings for carbonization. However, the moldings obtained by this method undergo considerable volumetric contraction of the resin in the burning stage, resulting in low density with inferior non-permeability to gases and in low physical strength when formed into a thin sheet of a thickness of 0.4-1.5 mm which is generally adopted for separation sheets in actual applications.
On the other hand, there have also been known various graphite type non-permeable carbonaceous moldings which is obtained, for example, by a method of filling an impregnant such as pitch, tar, resin or the like in pores of graphitic moldings obtained by bake-carbonization, and carbonizing the impregnant by a secondary baking operation. This method is, however, objectionable in that the bake-carbonized products are susceptible to cracking due to a difference in thermal contraction between the graphitic moldings and impregnant.
Further, disclosed in Japanese Laid-Open Patent specification No. 57-72273 is a method of molding graphite power with the use of a phenol resin solution and baking the resulting moltings at a high temperature to obtain a non-permeable carbonized product consisting substantially of graphite in its entirety. This method also involves the problem of cracking in the baking stage due to a difference in thermal contraction between graphite and binder, coupled with the necessity for impregnating the binder repeatedly in order to impart sufficient non-permeability to moldings, which invariably results in increases in the number of steps of the manufacturing process and in the production cost.